Light redirecting films are typically thin transparent optical films or substrates that redistribute the light passing through the films such that the distribution of the light exiting the films is directed more normal to the surface of the films. Typically, light redirecting films are provided with ordered prismatic grooves, lenticular grooves, or pyramids on the light exit surface of the films which change the angle of the film/air interface for light rays exiting the films and cause the components of the incident light distribution traveling in a plane perpendicular to the refracting surfaces of the grooves to be redistributed in a direction more normal to the surface of the films. Such light redirecting films are used, for example, to improve brightness in liquid crystal displays (LCD), laptop computers, word processors, avionic displays, cell phones, PDAs and the like to make the displays brighter.
Previous light redirecting films suffer from visible Moiré patterns when the light redirecting film is used with a liquid crystal or other display. The surface elements of the light redirecting film interact with other optical films utilized in backlight assemblies, the pattern of printed dots or three-dimensional elements on the back of the light guide plate, or the pixel pattern inside the liquid crystal section of the display to create Moiré, an undesirable effect. Methods known in the art for reducing Moiré have been to die cut the light redirecting films such that the lenticular array is not normal to any side of the sheet. This makes the lenticular array be at an angle relative to another light redirecting film or to the display electronics. Methods also used include randomizing the linear array by widths of the linear array elements, to vary the height along the linear array periodically, to add a diffusing layer on the opposite side of the linear array on the film, or to round the ridges of the linear array. The above techniques to reduce Moiré also cause a decrease in on-axis brightness or do not work to adequately solve the Moiré problem. Moiré and on-axis brightness tend to be related, meaning that a film with high on-axis gain would have high Moiré in a system. It would be beneficial to be able to reduce the Moiré while maintaining sufficient on-axis gain.
In addition, there are relatively few numbers of light redirecting films compared with the numbers of liquid crystal display configurations. Each display configuration was selected to fill a desired output. The amount of on-axis gain, viewing angle, Moiré reduction, and total light output were all tailored by combining different films in different configurations. The light redirecting film used in the systems is limited because there are only a few different light redirecting surface textures available. It would be desirable to have a light redirecting film that was customizable to the desired output of the display device.
Typical light directing films provide high on-axis illumination at the expense of illumination at angles between 40 and 90 degrees from the normal. These high, on-axis light directing films are useful for portable display devices such as laptop computers and games were a high on-axis brightness lessens the power consumption for batteries and provides for some level of viewing privacy. For some TV and monitor applications that are intended for public viewing, high brightness over a wide range of viewing angles allows for consistent viewing of images and video. It would be desirable to have a light directing film that could provide high brightness over a wide range of viewing angles.
U.S. Pat. No. 5,919,551 (Cobb, Jr. et al) claims a linear array film with variable pitch peaks and/or grooves to reduce the visibility of Moiré interference patterns. The pitch variations can be over groups of adjacent peaks and/or valleys or between adjacent pairs of peaks and/or valleys. While this varying of the pitch of the linear array elements does reduce Moiré, the linear elements of the film still interact with the dot pattern on the backlight light guide and the electronics inside the liquid crystal section of the display.
U.S. Pat. No. 6,354,709 discloses a film with a linear array that varies in height along its ridgeline and the ridgeline also moves side to side. While the film does redirect light and its varying height along the ridgeline slightly reduces Moiré, it would be desirable to have a film that significantly reduces the Moiré of the film when used in a system while maintaining a relatively high on-axis gain.
US Application 2001/0053075 (Parker et al.) discloses the use of individual surface structures for the redirecting of light to create high on-axis gain in a LCD device.
U.S. Pat. No. 6,721,102 (Bourdelais et al.) disclose a visible light diffuser formed with complex polymer lenses. The complex polymer lenses disclosed in U.S. Pat. No. 6,721,102 are created by adding micrometer sized polymer lenses on the surface of low aspect ratio polymer base lenses. The ratio of smaller lenses to large lenses is between 2:1 to 30:1. The diffuser disclosed in U.S. Pat. No. 6,721,102 is useful for diffusing light sources, in particular, LCD backlight sources.
U.S. Pat. No. 6,583,936 (Kaminsky et al) discloses a patterned roller for the micro-replication of light polymer diffusion lenses. The patterned roller is created by first bead blasting the roller with multiple sized particles, followed by a chroming process that creates micro-nodules. The manufacturing method for the roller is well suited for light diffusion lenses that are intended to diffuse incident light energy.
US Application 2005/00247554 (Epstein et al.) discloses surface structures that are coated with a matrix polymer contain polymer beads preferably having a diameter of between 2 and 5 micrometers to create random scattering.
US Application 2005/0047112 (Chen et al.) discloses a light guide plate with prisms formed on the surface of the light guide plate. The surface of the prisms contains a coated inorganic nano-particle layer consisting of titanium dioxide, silicone dioxide or aluminum oxide to scatter transmitted light.
US Application 2005/0140860 (Olczak) discloses an optical film defined by a first surface structure function modulated by a second surface structure such that the first surface acts to diffuse light incident on the film and the second surface also functions to diffuse incident light.
US Application 2005/0174646 (Cowan et al.) discloses a reflective diffuser, which transmits or reflects incident light into a specific range of angles.
U.S. Application 2002/0044351 (Nilsen) discloses a polarizer comprising sub-wavelength microstructures that are partially covered with a light transmissive inhibiting surface for polarizing light. The inhibiting surface can also comprise a reflective surface such as a metallized coating.
U.S. Pat. No. 6,077,560 (Moshrefzadeh et al) discloses a method of selectively printing a structured substrate without the use of a mask. The method includes coating the structured surface with a filler material such that the structured surface is covered.
U.S. Patent Application 2004/0012570 (Cross et al) discloses a resistive touch screen comprising a microstructured surface that is uniformly coated with an electrically conductive layer.
WO 98/50806 discloses a brightness enhancement article comprising a rounded prism structure and light scattering protrusions to increase the optical gain of incident light. It has been found that the scattering protrusions described in WO 98/50806 tends to result in unwanted abrasion of adjacent surfaces and the rounded prism tend to scratch resulting in unwanted cosmetic defects.